There is but one thing more to describe in the mechanism of the body—the connecting link between the last two sections. In the last we saw how the body receives stimuli from the external world; in the one before, that when these stimuli reach the central nervous canal it in turn stimulates the organs to perform such movements as circumstances require. What, therefore, remains to be described is the working of that canal by which these necessary movements are ordered and controlled. Now, in speaking of reflex action a few pages back, we said that the nerves which bring in stimuli from the periphery distribute them about the neural canal to those cells whose activity, by sending out fresh stimuli to the muscles, produces the requisite movements. These motor cells, however, are not scattered about the spinal cord anyhow. They are collected into clusters, or nuclei, as they are sometimes called, and each cluster has special duties—_i.e._, a special organ to control. Thus, we say that there are in the central nervous system centres—a nervous centre to control the leg; another to work the diaphragm; another for the muscles of the ribs; more for the arm, hand, etc. And these centres are in communication with one another, so that they may not pull different ways. In the first example of reflex action given in Section II. of this essay, the sensation of a pin-prick was first conveyed to the centres controlling the limb injured, by whose activity it was drawn away from the danger. But the nerve which gave the warning which produced this elementary movement distributed the impression that something was wrong to the higher centres, so that the whole body was involved in protecting, doctoring, and avenging the outraged member; from which it would appear that the lower centres are under control of higher ones. And this is the case. If we may be allowed the metaphor, there are captains of tens, who are under the direction of captains of fifties, and the captains of fifties receive their orders from captains of hundreds. The nerve canal, the manner of whose formation as a simple tube is shown in Diagrams 5 and 42, has therefore different functions in different parts, and this to such an extent that considerable differentiation in bulk and structure is produced. The neural canal may be roughly divided into two parts—a comparatively simple tube, running the greater part of the animal’s length, containing many centres from which nerves run to the organs they control; and a complicated bulbous enlargement at one end, with thickened walls, in which are the centres controlling those in the cord, and thereby managing not so much organs as the whole animal. The former is called the spinal cord, the latter the brain.